The present invention relates to communications apparatus and methods, and more particularly, to power amplifier monitoring and control apparatus and methods.
Communications apparatus often include a power amplifier that is used to amplify a signal to appropriate levels for transmission, for example, in free space or on a transmission line. For example, wireless terminals (e.g., cellular telephones and other wireless-capable devices) typically include a power amplifier that is used to amplify a radio frequency (RF) signal for transmission by an antenna.
Power amplifiers often encounter non-ideal reflection arising from impedance mismatches between the power amplifier and transmission medium. In the case of a wireless terminal power amplifier, for example, impedance mismatches may be caused by a variety of factors, including changes in surroundings caused by motion of the terminal and/or objects in the terminal's environment. Such changes may occur, for example, when the terminal is moved near a user's body or near reflective or shielding structures.
Impedance mismatches can significantly affect performance of a power amplifier. For example, impedance mismatch may cause the voltage standing wave ratio (VSWR) presented to the power amplifier to be high enough to cause damaging voltages and/or currents at the power amplifier's output. In addition, large reflected power and high VSWR may lead to performance-degrading phenomena, such as AM/AM and AM/PM distortion.
Conventional solutions for avoiding or reducing such problems include reducing mismatch and/or reducing its damaging or degrading effects. In a wireless terminal, for example, mismatch can be reduced by using an isolator or a balanced antenna circuit. Other conventional approaches attempt to detect when undesirable or dangerous VSWR or reflected power conditions are present (or are about to be present), and take corrective and/or protective actions that can prevent damage or reduce performance degradation.